Commercially available dental composite resins exhibit the inherent problem of 2-5% volumetric shrinkage during the polymerization process. This polymerization creates contraction stress in the composite restoration, which can disrupt the marginal seal between the composite and the tooth structure. Despite the development of new dentin bonding agents, no system is able to currently withstand the formation of gaps at the tooth/restoration interface, due to unavoidable polymerization contraction stress. In addition, available composite resins suffer from low toughness, which results in marginal and bulk fracture of composite restorations. [unreadable] [unreadable] We describe here a novel means of reducing contraction stress as well as improving toughness in composite resins that entails only two modifications of existing technology. In the first, a compliant phase material, such as polybutadiene, in aggregate form with fumed silica, replaces the reinforcing filler. These homogeneously dispersed, compliant aggregates yield under the hydrostatic tensile stress state generated during polymerization and allow expansion, or compliance, of the aggregates, thereby offsetting cure shrinkage and reducing stress at the bonded interfaces. [unreadable] [unreadable] The second modification is the introduction of a small amount of plasticizer into the matrix monomer. Plasticizers reduce the glass transition temperature (Tg) during curing such that chain mobility remains high for a longer time during the course of the crosslinking and chain extension reaction, and thereby allows greater compliant offset of cure shrinkage and reduction of contraction stress as monomer conversion to polymer progresses. [unreadable] [unreadable] We show that the presence of micrometer-sized compliant aggregates results in increased flexural modulus, flexural strength and energy to break, and that micrometer-sized aggregates in combination with plasticizer result in reduced polymerization contraction stress. In addition, we show that nanometer-sized aggregates, which present a very large compliant surface area to the resin matrix, result in a proportionately increased reduction in polymerization contraction stress. [unreadable] [unreadable] Overall, compliant aggregates have the potential to circumvent two of the major problems of today's esthetic and posterior resins, problematic contraction stress and low toughness. As a modification to commercially available composite resins, compliant aggregates have the potential to provide a superior composite restorative system. [unreadable] [unreadable] [unreadable]